ER 's George Clooney, Julianna Margulies and More Spill On-Set Secrets: 9 Revelations from Reunion

The cast of ER reunited 12 years after concluding the show's 15-season run on NBC in 2009 — and it was well worth the wait!

On Thursday, the OG County General crew made a virtual appearance on Stars in the House to support costar Gloria Reuben's (Jeanie Boulet) organization, Waterkeeper Alliance. The non-profit works to ensure that communities around the world have clean water.

The special Earth Day episode — which streamed exclusively on PEOPLE's social platforms — saw former cast members George Clooney (Doug Ross), Julianna Margulies (Carol Hathaway), Noah Wyle (John Carter), Dr. Laura Innes (Kerry Weaver), Anthony Edwards (Mark Greene), Goran Visnjic (Luka Kovac), Ming-Na Wen (Jing-Mei Chen), Laura Cerón (Chuny Marquez), Conni Marie Brazelton (Connie Oligario), CCH Pounder (Angela Hicks), Ellen Crawford (Lydia Wright) and Reuben all catch up together live. The cast reminisced over their positive experiences working on the medical drama and shared several behind-the-scenes memories.

Here are the biggest revelations:

What the cast misses most about the show

While reminiscing about the glory days, Edwards, 58, began by noting what a "special time" it was for the cast. "It's one of those things where the sanity and the happiness and the joy was on the set, and all the madness was around," he said. "But when you came to work and when you played with all of these wonderful people, we laughed nonstop. The image I always have is we were on stage 11, and we were just laughing. … Next door at Friends, you go onto that stage and it was like a cemetery. There was no laughing, there was nothing."

Edwards added, "I feel like I laughed and stayed in love for eight years. It's all of these people that I love and miss."

Clooney, in turn, noted that was "the secret to the whole thing." He continued, "We had fun because we were like, 'Wow, this is really crazy!' We know how lucky we are and we knew that would never happen again."

RELATED: George Clooney 'Laid Out the ABCs of Professionalism' on ER Set, Says Costar Noah Wyle

Noah Wyle had a unique way of practicing his scenes

As a way to practice technique for his ER scenes, Clooney revealed that Wyle was "stitching a pig's foot" off-camera.

Sven Arnstein/NBCU Cast of ER

Dr. Laura Innes once accidentally switched her character's cane from one hand to the other

When Clooney asked Innes, 63, whether she ever switched her cane from one arm to the other, she admitted that it happened "one time" by "accident."

"I was walking down the hall and I was like, 'Oh, s---.' I started doing something with the wrong hand," she recalled. "So, I didn't tell anyone. I didn't say, 'Let's do another take.' I was afraid of getting into trouble, so I just went. 'Oh well!'"

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George Clooney's wife, Amal Clooney, doesn't like his ER character

Clooney, 59, revealed that wife Amal, 43, has a bone to pick with the actor's ER character after she began watching the series for the first time amid the COVID-19 pandemic.

"This has been a very, very disastrous thing for me because I forgot all of the terrible things I'd done as Dr. Ross," the Oscar winner said. "My wife keeps going, 'Is that it? Are you done? Season 3 — do you finally settle down with Nurse Hathaway?' It's been a disaster for my marriage," he joked.

The cast supported Anthony Edwards during his Saturday Night Live hosting gig

The County General bunch cared about one another so much that they made the trip to N.Y.C. when Edwards hosted Saturday Night Live in 1995.

"Remember [when] Tony was hosting Saturday Night Live and we all got on a plane and flew out for the night because we just wanted to be there for him when he did it?" Clooney recalled.

RELATED: Julianna Margulies Admits She and George Clooney Had a 'Crush on Each Other' During ER

getty images George Clooney on ER

How Julianna Margulies went from ER guest star to series regular

The Good Wife alum's ER character was supposed to originally be killed off, but she explained what transpired behind-the-scenes that resulted in her guest-starring part turning into a full-time role.

"George called me. He said, 'If you're thinking about taking a job, I urge you not to. I think in the next couple of days, you might be offered a series regular role,'" she recalled. "And I was like, 'But I died! How does that work?' He was like, 'I don't know, but I think it's going to work.'"

She continued, "I was about to go do Homicide: Life on the Street because I had done two episodes of it before I did the pilot of ER and they wanted me back. I didn't take the job. And a week later, they called and said, 'We're going to make you a series regular.'"

Clooney noted that during the test screening, the network realized that Margulies' character was "an important part" of the positive reaction that the show received. "There was no way they were going to kill her off," he said.

George Clooney reflected on his late aunt, Rosemary Clooney, appearing on the show

The cast reminisced over Rosemary's 1995 guest-starring part on ER, for which she received a Primetime Emmy Award nomination. She played an Alzheimer's patient who communicated through singing.

"It's very hard to play less intelligent than you are or more confused than you are, and she had a really interesting quality. And then when she began to sing, the expectation is that she would fall into her old sort of performance rhythm, but she didn't," Wyle, 49, recalled. "She found a sort of vocal quality that seemed much more pedestrian and less polished than a professional would be, which made it all the more heartbreaking."

Clooney then joked, "Noah was the nephew she always wanted."

After the cast watched the scene where Rosemary's character was comforted by Wyle's, Clooney said: "I miss her. She was the fun one in my family."

Getty Images Cast of ER

The cast isn't sure about an ER reboot

While discussing the possibility of an ER revival or reboot series, Clooney suggested that it might be hard to recapture what made the show so good.

"I don't know. The hardest part is that when you look at the show and consistently over so many years — it would be hard to say that you could do it at the level that we did it. I'm not sure that that's available," he said, noting that "it's hard to catch lightning again."

Margulies, 54, added, "You can't capture lightning in a bottle twice. I think you have to leave what was so beautiful and move on, because it just feels cheap. … It would cheapen it for me."

George Clooney compared ER to his time on Roseanne

The actor contrasted his positive experience being on ER to working on shows that "were really miserable" for him.

While discussing being on the first season of Roseanne, he said: "Oh my God. There was some crazy s--- that went on there."